The Britax scores slightly better in testing from what I've heard, it also provides a bit more room for the child widthways, better leg room for child due to the anti-rebound bar, fabrics are nicer, and the pads on the harness aren't as bulky as they are on the Joie which makes it a bit easier to adjust them. I also find it fits a bit more stable in most cars because the anti rebound bar is adjustable which gives it a really solid, secure fit.
If you'll use it for any future children be aware the Britax M doesn't come with a newborn insert and doesn't really fit children well until about 6 months old. The Joie is usable from birth.
The advantages of the Joie are that the headrest goes up a bit further, meaning you'll likely get more time out of it height wise. It also has no height limit as it's not i-size. You've probably outgrown the first two parts of the insert, but you can use the wedge part to get a bit of extra recline if you want to (it's not generally necessary at this age.) It sits right up against the backrest of the vehicle seat, which means not much child legroom, and can make the rotate handle hard to reach and in some cars it's stiff to recline/rotate, especially bucket type seats due to the pressure against the seat back. However, this makes it extremely compact, and none of this is a safety issue, just convenience (children generally don't mind crossing their legs or resting them up the seat back) so if you need front seat space, choose the Joie.
The Joie can be wobbly in some cars (this isn't a safety issue, as long as all indicators are green and the seat makes contact with the vehicle seat).